The invention relates to a radio transmission system for simultaneously transmitting a plurality of information signals, the system comprising a network of transmitters operating with substantially the same carrier frequencies and receiving the said information signals synchronously.
Such a system may be used, for example, for private networks for transmitting information signals to mobile receivers or for the transmission of several high quality radio programs to a receiving area. As known, transmitter networks which were characterized by different carrier frequencies in the VHF range, which carrier frequencies corresponded with the programs to be transmitted have always been used sofar for this purpose. The greatest drawback of this technique is that it gives rise to a high spectral congestion.
To obviate this drawback, the invention proposes to transmit several information signals, that is to say several programs in the above-mentioned example, by a network of transmitters which operate with substantially the same carrier frequencies, for example 100 MHz.
However, such a system causes some problems as regards the reception. One problem is caused by interferences between VHF signals of the same frequency, which are received from several transmitters. In receiving areas where the different signals are received with levels which are very near to one another, these interferences may result in an almost complete disappearance of the total signal received by the receiver. It should furthermore be noted that this phenomenon, denoted "fading" hereinafter, also depends for a mobile receiver on the Doppler effect. A further problem is caused by the fact that, even if precautions have been taken to apply the same information signals synchronously to the different transmitters of the network, a receiver does not synchronously receive these signals, particularly from the two transmitters nearest to the receiver, because of the difference in propagation time of the carrier signals. The same information signals coming from the two nearest transmitters, which have been subjected to different delays, then overlap. These overlaps are not annoying when the received carrier signals have different levels. In contrast therewith, in zones where the carrier signal levels are near to one another, the quality of the reception deteriorates. This deterioration is manifested by serious distortions when the transmitted signals are analog signals and by a high error probability when the transmitted signals are data signals.
A known means to solve these problems is described in U.S. patent application Ser. No. 938,046, filed Aug. 30, 1978. This solution is based on the diversity technique, and consists of the transmission of at least twice the same information signal and by having the ratio between the transmitter powers of at least two transmitters vary between an information signal transmission and the next transmission of this information signal. In this manner, the position of the zones where the reception is poor is varied, so that a random receiver receives alternately a high grade information signal.